Cachexia is a syndrome characterized by the wasting of tissue mass in diseased animals, and is grossly reflected as a loss of host weight. Cachexia is a progressive and often fatal complication found in many different chronic disease states and its consequences require that the goals of therapy should not be solely to redress the underlying disease. The loss of protein stores, loss of body weight and generally poor nutritional status of cachectic patients can be independent sources of morbidity and mortality. Also, the debilitation associated with cachexia is a significant limitation on the patient's ability to tolerate aggressive medical and surgical therapies which are directed to the primary etiology.
2.1. THE SIGNS OF CACHEXIA ARE DISTINCT FROM THOSE OF STARVATION
Cachexia is a severe, often life-threatening complication commonly encountered in association with a variety of insults: cancer, chemotherapy, radiation injury, chronic infection, trauma and surgical stress. Food intake insufficient to meet the total energy needs of the host is a constant element of the cachectic syndrome. In addition to this relative hypophagia which is a defining feature of cachexia, anorexia is also frequently encountered.
However, studies of the syndrome indicate that cachexia is not simply due to a dietary intake of protein and carbohydrate below the needs of the host. Cachexia differs from unstressed caloric deprivation in that the pattern of wasting seen during partial or complete starvation is associated with an initial whole body lipid loss concurrent with a relative conservation of tissue protein. By contrast, cachexia is characterized by the significant loss of both lipid and protein from tissue reservoirs
2.2. THE IMPLICATIONS OF THE DIFFERENCES BETWEEN CACHEXIA AND STARVATION
The most commonly accepted general explanation for cachexia is that the host's proteins are broken down in the tissues to provide a source of amino acids. These amino acids in turn are thought to be needed for the synthesis of glucose, albumin and host defense proteins in the liver.
This well-accepted theory suggests that therapies directed toward increasing the total intake of calories and proteins should substantially ameliorate the cachectic syndrome. However, even as drastic an intervention as total parenteral nutrition is not able to effectively treat cachexia. (Brennan, 1986 NEJM 305:375, Detsky, et al., 1987, ANN INTERN MED 107:195, McGeer, et al., 1989, ANN INTERN MED 110:734, Koretz, 1984, J CLIN ONCOL 2:534).
In addition to the failure of supplementary nutrition as a therapeutic modality, the fundamental difference in the pattern of the losses of lipid and protein between cachexia and starvation also indicates that cachexia is neither the result solely of the abnormally increased nutritional needs due to the underlying disease nor of anorexia due to the disease's disruption of the physiologic regulation of appetite. Rather the differences suggest the presence of some fundamental changes in the host's metabolism due, directly or indirectly, to the underlying disease.
Further supporting this conclusion has been an accumulation of evidence implicating soluble host-produced regulatory and effector proteins, known as cytokines, in the chain of events which leads to cachexia. Experiments have been conducted in which the blood circulation of a normal and a cachectic animal were joined In these experiments with so-called "parabiosed" animals, it was observed that the otherwise normal animal rapidly developed cachexia although the underlying disease remains entirely with the original host. These and other observations strongly implicate circulating mediators as the proximal cause of cachexia, i.e., this catabolic condition is not the passive result of the excessive metabolic demands imposed by the growth of the invading cells or organisms, nor the simple result of a lesser food intake than that required to meet metabolic demands.
One possibility as to the identity of these humoral factors was that the soluble mediators produced in cachexia were the same molecules as had been already identified as host immune/inflammatory-related molecules (cytokines), and shown to be secreted by lymphocytes and macrophages. The theory that these cytokines were involved in cachexia was confirmed by the observation that the administration of exogenous Tumor Necrosis Factor (also known as cachectin, herein abbreviated "TNF") to test animals mimicked many features of cachexia. (Darling et al., 1990, CANCER RES 50:4008; Beutler & Cerami, 1988, ADV IMMUNOL 42:213). Further, anti-TNF antisera are able to ameliorate many, but not all, the signs of cachexia in experimental tumor systems. (Sherry, et al., 1989, FASEB J 3:1956; Langstein, et al., 1989, SURG FORUM 15:408).
2.3. THE IMPACT OF THE ROLE OF CYTOKINES IN CACHEXIA ON THE SEARCH FOR THERAPIES
It should be clear that even if all aspects of the cachectic syndrome were attributable to some single cytokine or, alternatively, to the activity of some combination of several cytokines, hormones and other humoral factors, such knowledge would not in and of itself provide a cellular or biochemical mechanism to explain the metabolic changes that underlie cachexia, nor lead directly to an effective therapy. Ultimately, cytokines must interact with target cells and induce metabolic or phenotypic changes in their targets to be of physiological and pathophysiological significance. Thus the general identification of cytokine mediation and the specific implications of particular cytokine mediators are only intermediate objectives in the determination of precisely what cellular and/or systemic metabolic changes occur to bring about the full cachectic picture.
The findings that implicate cytokines as mediators of the various cachectic syndromes, combined with the widespread focus on liver and peripheral muscle as major contributors to the metabolic changes of cachexia prompted many to look at the effects of known cytokines on liver and muscle cells and to search for new cytokines having an effect on these tissues. To date, however, no known cytokines, individually or in combination, have been shown to directly mobilize amino acids from protein stores in in vitro systems using cells typical of these presumed sites of protein breakdown in vivo.
2.4. NITRIC OXIDE AS A MEDIATOR OF ENDOTOXIC SHOCK
Nitric oxide (NO), a molecule produced enzymatically from L-arginine by nitric oxide synthase (NOS), is a mediator of both physiological homeostasis and inflammatory cytotoxicity. Moncada, S. & Higgs, A., 1993, THE NEW ENGLAND JOURNAL OF MEDICINE 329, 2001-2012; Nathan, C., 1992, FASEB J 6, 3051-3064. NO production via the constitutive and inducible isoforms of NOS in endothelial cells for instance, causes vasodilation and governs blood pressure and tissue perfusion. Kilbourn, R. G., Jubran, A., Gross, S. S., et al., 1990, BIOCHEM. BIOPHYS. RES. COMMUN. 172, 1132-1138. NO production by an inducible NOS in activated macrophages, on the other hand, confers cytotoxic, increases vascular permeability, and enhances the release of TNF.alpha. and IL-1. Ding, A. H., Nathan, C. F. & Stuehr, D. J. J Immunol. 141, 2407-2412 (1988); Kubes, P. & Granger, D. N. Am. J. Physiol. 262, H611-H615 (1992); Van Dervort, A. L., Yan, L., Madara, P. J., et al. J Immunol. 152, 4102-4109 (1994); Bouskela, E. & Rubanyi, G. M. SHOCK 1, 347-353 (1994); Hibbs, J. B., Taintor, R. R. & Vavrin, Z. Science 235, 473-476 (1987); Granger, D. L., Hibbs, J. B., Perfect, J. R. & Durack, D. T. J. Clin. Invest. 85, 264-273 (1990). Insight into the diverse biological actions of NO has been facilitated by compounds that interfere with NOS to inhibit production of NO. Because the NOS isoforms are highly conserved, however, the available NOS inhibitors have not been found to discriminate significantly between the activities of constitutive versus inducible NOS.
Previously available NOS inhibitors have had limited success in improving survival from endotoxemia, in part because they indiscriminately suppress endothelial-derived relaxing factor (EDRF), Cobb, J. P., et al., 1992, J. EXP. MED. 176:1175-1182; Minnard, E. A., et al., 1994, ARCH SURG. 129:142-148; Billiar, T. R., 1990, et al., J LEUKOCYTE BIOL. 48:565-569. Suppression of EDRF during endotoxemia may impair survival by causing vasoconstriction and a diminution of blood flow to critical vascular beds. Hertofore there have been reported no compounds that inhibit cytokine-inducible macrophage NO without also inhibiting endothelial-derived NO.